Motor vehicle exhaust is of great concern to the public because it is a major cause of atmospheric pollution in many areas of the country. The size of the particles in a motor vehicle exhaust plume is a significant factor in the human health impact of exhaust emissions. Particles that have a diameter less than 2.5 micron are especially harmful.
Exhaust emission opacity measures the density of particles in a vehicle's exhaust plume. Opacity meters are used to measure the opacity of the exhaust plume. To measure opacity, a beam of light is propagated across the exhaust plume of a vehicle. A light detector is positioned at the other side of the exhaust plume to measure the decrease in intensity of the light due to the particles in the exhaust plume. If there are no particles, the opacity meter will indicate zero percent opacity. As the density of particles in the exhaust increases, so does the opacity.
A disadvantage of typical opacity meters is that they must have a constant path length for the light beam that propagates through the exhaust. Also, the exhaust plume size must not change during measurement. This limits the use of typical opacity meters to direct tailpipe measurement; they cannot be used as a remote sensing tool to measure the opacity of a vehicle exhaust plume of uniknown physical extent or dilution.
Another disadvantage of typical opacity meters is that they only measure the relative density of the particles, and cannot predict anything about the particle size. The very small diameter particles that are especially harmful to humans are usually invisible to the human eye. An exhaust plume that may appear “clean” to the human eye because there is no visible black emission may contain harmful sub-micron particles. In addition, almost all commercially available opacity meters use green light that has a wavelength of about 550 nanometers. As a consequence, particles that a have diameter of less than 550 nanometers will not be detected. If an exhaust plume is filled with very small particles about 200 nanometers or less, a conventional opacity meter will not measure the small particles and will report zero opacity. Yet, the small particles are very dangerous to human health.
U.S. Pat. No. 6,025,920 discloses an apparatus and method for measuring the opacity of an exhaust plume of a diesel locomotive. A single monochromatic light source is passed through the exhaust plume of a locomotive and the decrease in intensity is determined using a detector. The opacity is calculated from the decrease in intensity. This system uses infrared and ultraviolet light of relatively large wavelengths, and thus provides no information about the very small particles that are especially harmful for humans.
U.S. Pat. No. 5,257,087 discloses an apparatus and method for measuring the amount of small and large particles in a fluid contained in a test cell. The fluid is either water or chemicals that are used in the semiconductor fabrication process. The sizes of the particles are measured in order to test for impurities. A laser sends a light pulse through a test cell and a pulse counter determines, from the intensity of the light scattered by the particles, whether the particles are large or small.
U.S. Pat. Nos. 5,319,199 and 5,498,872 disclose a method and apparatus for measuring the amount of specific gas in a vehicle exhaust. Ultraviolet light is propagated across the exhaust plume of a vehicle and a detector determines the reduction in intensity of the ultraviolet light and computes the amount of nitric oxide in the exhaust. Infrared light is used to determine the amount of hydrocarbons, carbon dioxide and carbon monoxide in the exhaust. The opacity and the size of the various particles in the exhaust plume are not determined.
U.S. Pat. No. 5,835,211 discloses a single particle optical sensor for measuring the size of a particle in a liquid or gas. A light extinction method is used to determine the size of a particle that is over 1.5 μm and a light scattering method is used to determine the size of a particle as small as 0.1 μm.
Consequently, there is a need for an improved apparatus and method for determining the opacity of a vehicle exhaust plume of unknown physical plume size that accounts for small size particles in the exhaust, and for determining the average particle size in the exhaust.